Optical signals are distorted as they propagate in fluctuating media like atmospheric turbulence. A system to correct the distortion, known as adaptive optics (AO), is employed to optimize system performances in the presence of the random disturbances. Such adaptive wavefront correction is usually achieved by introducing a spatially varying pattern of optical path differences across the aperture of the receiving optical system using deformable Mirror (DM) technology. As a critical element to such systems, deformable mirrors offer the greatest promises for high performance wavefront correction. Conventional deformable mirrors use multiple bulk piezoelectric actuators or magnetic actuators to deform mirrors. While these mirrors offer high accuracy and are capable of conforming to wavefront distortion associated with broad or narrow band spectrums, they tend to be bulky, heavy, expensive, and typically operate with less than 1000 actuators and at relative slow speed. An emerging generation of deformable mirror (DM) technologies based on Micro-Electro-Mechanical Systems (MEMS) manufacturing is promising to lead to DM components with capabilities exceeding those of conventional DMs while, at the same time, reducing cost, weight, and power electronics requirements. MEMS-based deformable mirror technologies, especially those based on electrostatic actuators, have been successful in small stroke applications. However, large stroke deformable mirror systems using MEMS actuators have not yet demonstrated sufficient attractive combinations of high stroke, low voltage, and high system reliability.
In general, there are two dominant types of deformable mirrors: segmented deformable mirrors and continuous-face-sheet deformable mirrors. A segmented mirror does not have a continuous mirror surface but has individually controllable tip, tilt, and piston motion on each mirror segment. These mirrors have the advantage of segments being independently controllable. Its primary drawback is the gap between mirror segments that can scatter and diffract light in an undesired and uncontrolled manner. The drawback can be partially avoided by using the second type of deformable mirror that has a continuous faceplate with an array of actuators attached underneath. The actuators can be designed to push and/or pull the mirror surface. Since the surface is continuous, there is some mechanical crosstalk (or influence function) coupled from one actuator to its adjacent mirror members. Thus, an optimized mechanical design is usually in need to minimize the crosstalk, and a computer algorithm may be developed to factor in the effects of influence function into the mirror control signals.
With regard to the continuous-face-sheet deformable mirror, the U.S. Pat. No. 6,384,952 to Clark et al. (2002), incorporated herein by this reference, discloses a continuous-face-sheet DM that employs a mirrored membrane fabricated, for example, from metal-coated silicon nitride and actuated by an array of vertical comb electrostatic actuators that are disposed underneath the membrane. Use of vertical comb actuators can provide larger stroke for a given applied voltage than the parallel plate electrostatic actuators in other continuous-face-sheet designs. However, this design requires placing vertical two teeth sets precisely relative to each other, one on the substrate and the other suspended on a membrane member, respectively, thus is unduly complicated in manufacturing the DM structure. Moreover, because of the electrostatic actuation, the device does not offer sufficient actuation force to meet the stringent requirements of a deformable mirror device for adaptive optics applications.
U.S. Pat. No. 7,336,412 to Yang (2008), also incorporated herein by this reference, describes a micro-controllable continuous-face-sheet deformable mirror comprising a mirror membrane and under which a plurality of controllable piezoelectric microactuators is coupled to the mirror membrane. Each piezoelectric microactuators comprises a pedestal, a piezoelectric microactuator, and a supporting substrate. The piezoelectric actuator structure is mounted on the supporting substrate and has electrodes defined on opposing surfaces so that in-plane stresses electrically induced in its piezoelectric layer cause the actuator membrane to bend out of the unstressed plane in a selected direction. In this prior invention, the pedestal is connected to the mirror membrane to couple deformation of the piezoelectric actuator into substantially local deformation of the mirror membrane. However, because each of the piezoelectric actuator is mechanically a continuous membrane structure having the pedestal located at or near the membrane center, the bending deformation of the piezoelectric actuator induced by an in-plane stresses is significantly restricted. Furthermore, because of its thin film deposition method in manufacturing the deformable mirror, the reliability of the mirror system is not satisfactory.
On the other hand, with regard to the segmented deformable mirrors, the U.S. Pat. No. 7,019,434 to Helmbrecht (2006), incorporated herein by this reference, discloses an electrostatically-actuated segmented mirror apparatus comprising a substrate and the segmented mirror array elevated above the substrate and supported by curved flexures. While use of the curved flexures enables high stroke, the mirror operation in this prior invention still suffers high voltage actuation. In addition, due to the film deposition process in use, residual gradient stress unavoidably builds up through the actuator thickness and the flexure, inducing multiple problems to the actuator including bowing, tilting, position shift in the long run, and sensitivity to the thermal variations. Although efforts can be taken to optimize the residual stress, the problem remains a significant concern. In fact, stress engineering in thin-film micromachining is usually a time-consuming and labor-intensive procedure. Being sensitive to film geometry and thickness designs, the optimized recipe is neither fully transferrable nor scalable to a new actuator design that may have a modified dimension set, film aspect ratio, and/or layout geometry.